1. Field of the Invention
The invention relates to a drive for signal processing equipment, particularly disk storage devices which include a direct drive brushless D.C. motor which has a rotor with a permanent magnet formed of at least two pairs of poles, means providing a field strength increase adjacent one of the magnet poles and causing a flux discontinuity, at least one stationary magnetic field sensor for detecting the rotor position, the sensor having an output signal with a zero position and being located in the field of the permanent magnet, a commutating signal generator stage, a commutating device controlled by the commutating signal generator stage, a stator winding connected electrically to the commutating device, the commutating signal generator stage being provided with a commutating comparator connected to the magnetic field sensor output, the commutating comparator having two switching points close to the portion of the magnetic field sensor output signal which is not affected by the increased field strength, an index signal generator stage which generates one index signal per rotor rotation in response to the flux discontinuity, the index signal generator stage including an index comparator having two switching points, a first of which is close to the portion of the magnetic field sensor output signal which corresponds to the field strength increase.
2. Description of the Prior Art
Such is known from U.S. Pat. No. 4,430,603, whose disclosure is hereby incorporated by reference. In the known arrangement, the precision of the index signal which can be achieved often leaves much to be desired. The term index signal precision is understood to mean that the index signal occurs always precisely when the rotor assumes a predetermined position relative to the stator. In particular in the known solution, changes in the magnetic field amplitude, for example resulting from manufacturing tolerances; the temperature effect of the magnetic field sensor of the index signal generator stage; and wear and tear or similar causes can lead to changes in the index signal so that the index signal no longer precisely coincides with the predetermined position between rotor and stator (hereinafter simply called zero position). This is extremely detrimental, for instance, when using the drive for hard disks where an index signal is required to exactly locate the start of a track.
The invention provides a drive of the aforementioned type which ensures increased precision of the index signal. Another object of the invention is the provision of a simple and inexpensive construction. According to the invention, these problems are solved by also connecting the index comparator to a magnetic field sensor, by having the second switching point of the index comparator coincide at least approximately with the zero position of the magnetic field sensor output signal and by using the output pulse edge or slope of the index comparator, which occurs immediately upon the occurrence of the second switching point of the index comparator, as an index indicator.